Anxiety disorders are common, chronic, costly, debilitating to quality of life, and are more prevalent than any other class of disorders in every country in the world where surveys have been taken. Deepening understanding of the nature of anxiety and related emotional disorders during the last decade has revealed that commonalities in etiology and latent structure among these disorders supersedes differences. At the same time, examination of extant single diagnosis psychological treatment protocols (SDPs) for these disorders underscores mechanistic similarities. These findings suggested the possibility of distilling a set of psychological procedures that would comprise an innovative Unified Protocol for Transdiagnostic Treatment of Emotional Disorders (UP), and this protocol has now been developed. If efficacious, the UP may represent a more efficient and possibly more effective strategy which would render treatment implications of comorbidity, not otherwise specified (NOS) and subdefinitional threshold anxiety disorder conditions moot. We now propose an evaluation of the efficacy of the UP in a group of patients with heterogeneous anxiety disorders by way of rigorous comparisons to existing evidence based SDPs benchmarked against a wait list control condition, using both statistical equivalence and superiority analyses. Additional aims include determining the durability of the UP relative to comparison conditions after treatment discontinuation, and ascertaining the differential impact of treatments on disorder specific symptoms vs. higher-order temperamental variables. Further analyses will indicate if changes in these higher order temperamental variables mediate long-term outcome as preliminary data suggests, and if this mechanism of action differs among treatments. PUBLIC HEALTH RELEVANCE: This study addresses an important public health issue by evaluating a novel and promising new transdiagnostic treatment strategy for common and disabling mental disorders, specifically the anxiety disorders. This information will answer important theoretical questions about the mechanism of treatment change, and might lead to more efficacious, cost-effective, and easily disseminable treatment strategies for some of the most prevalent mental disorders.